Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and is favorable for image pickup apparatuses such as television cameras for broadcasting, home-use video cameras, digital still cameras, and analog cameras, for example.
Description of the Related Art
In recent years, a zoom lens with a large aperture ratio, a high zoom ratio, and also high optical performance has been demanded for image pickup apparatuses. A positive lead type zoom lens in which a lens group having a positive refractive power is disposed foremost on the object side has been known as a zoom lens with a large aperture ratio and a high zoom ratio.
For example, Japanese Patent Application Laid-Open No. 2003-241096 and Japanese Patent Application Laid-Open No. 2011-107693 disclose five-group zoom lenses each including a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a negative refractive power, a fourth lens group having a positive refractive power, and a fifth lens group having a positive refractive power in this order from the object side toward the image side. The disclosed zoom lens is a small zoom lens with a high zoom ratio configured such that, during zooming, the first lens group and the fifth lens group do not move while the second lens group, the third lens group, and the fourth lens group move to vary the magnification and correct an image plane variation resulting from the magnification varying.
In the case of the above positive lead type zoom lens, it is relatively easy to reduce the size of the entire system and also to increase the zoom ratio. However, the higher the zoom ratio, the greater the variations in aberrations during zooming, and therefore the more difficult it is to obtain high optical performance over the entire zoom range.
To obtain high optical performance over the entire zoom range, it is important to properly set, for example, the refractive powers of the lens groups which move during zooming and the conditions for their movements, in particular, the lens configurations of the lens group for magnification varying, the conditions for their movements during zooming, and so on. Failing to properly set these configurations increases the variations in aberrations during zooming, and thereby makes it difficult to obtain high optical performance over the entire zoom range with a high zoom ratio.